A movable hearth furnace (a heating furnace) is used for manufacturing a reduced metal (a product) by heating and reducing a metal oxide (a raw material) containing a carbonaceous reducing material. Such a movable hearth furnace has a leveling screw for laying the raw material evenly on the hearth of the movable hearth furnace and has a discharging screw for discharging the product from the furnace. When the thickness of the raw material is changed according to the conditions of the operation or when deposit on the hearth of the movable hearth furnace is removed, the leveling screw and the discharging screw are necessarily lifted during the operation.
In the case that the leveling screw and the discharging screw are installed inside the heating furnace, a driving device of the screws is generally arranged outside the furnace in order to protect the driving device from a high-temperature atmosphere of the heating furnace. Therefore, a hole is formed in a side wall of the heating furnace, and a driving shaft extends to the outside of the furnace through the hole. Since a gap formed between the hole and the driving shaft causes an outburst of atmosphere gas in the furnace or an incursion of the air into the furnace, a sealing structure for preventing the problems is required.
When such a screw type device is provided with a lifting device, the relative position between the hole and the driving shaft is changed by lifting the screws. Therefore, the sealing structure should be able to follow the change in the relative position between the hole and the driving shaft.
In some cases, the leveling screw having the lifting device and the discharging screw having the lifting device in the furnace may be supported by the lifting device installed outside the furnace so as to be liftable. However, in such a structure, the hole formed in the side wall of the heating furnace and the driving shaft of the screw cannot be moved in the vertical direction and the mechanism for lifting the screw during an operation is not disclosed.